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LM
I've re-read this thread, I may have read it wrong earlier. If it's a water based levelling compound that was used in the first place,'which it appears to be' then two things have probably went wrong. 1- no primer used, 2- too weak of a mix. Either or both of these factors will cause a failure with that type of leveller, in which case any simple force such as a hammer lightly applied or a heavy scrapper should debond the poor leveller from the substrate.
When we've laid rubber floors in particular in the past to hostile substrates we've had to remove contaminated existing leveller from sand and cement screeds, one of the easiest ways of doing this is with a 'GOOD' scrabbler that has depth adjustment! It does cost a bit to hire such a machine but without doubt it's the fastest method. If I was under pressure for time it's the way I would go, then again I've experience in doing this! Sometimes time is money! But if not simply chip away at it 🙁
When we've laid rubber floors in particular in the past to hostile substrates we've had to remove contaminated existing leveller from sand and cement screeds, one of the easiest ways of doing this is with a 'GOOD' scrabbler that has depth adjustment! It does cost a bit to hire such a machine but without doubt it's the fastest method. If I was under pressure for time it's the way I would go, then again I've experience in doing this! Sometimes time is money! But if not simply chip away at it 🙁